diff --git a/.changeset/fix-readonly-scope-enforcement.md b/.changeset/fix-readonly-scope-enforcement.md
new file mode 100644
index 00000000..e35b967a
--- /dev/null
+++ b/.changeset/fix-readonly-scope-enforcement.md
@@ -0,0 +1,7 @@
+---
+"@googleworkspace/cli": patch
+---
+
+fix(auth): enforce readonly scopes by revoking stale tokens on scope change and adding client-side guard
+
+Fixes #168
diff --git a/src/auth_commands.rs b/src/auth_commands.rs
index f51ba6dd..ea12de13 100644
--- a/src/auth_commands.rs
+++ b/src/auth_commands.rs
@@ -128,6 +128,54 @@ fn token_cache_path() -> PathBuf {
     config_dir().join("token_cache.json")
 }
 
+fn scopes_path() -> PathBuf {
+    config_dir().join("scopes.json")
+}
+
+/// Save the configured scope set so scope changes can be detected across sessions.
+fn save_scopes(scopes: &[String]) -> Result<(), GwsError> {
+    let json = serde_json::to_string_pretty(scopes)
+        .map_err(|e| GwsError::Validation(format!("Failed to serialize scopes: {e}")))?;
+    crate::fs_util::atomic_write(&scopes_path(), json.as_bytes())
+        .map_err(|e| GwsError::Validation(format!("Failed to save scopes file: {e}")))?;
+    Ok(())
+}
+
+/// Load the previously saved scope set, if any.
+pub fn load_saved_scopes() -> Option<Vec<String>> {
+    let data = std::fs::read_to_string(scopes_path()).ok()?;
+    serde_json::from_str(&data).ok()
+}
+
+/// Returns true if a scope does not grant write access (identity or .readonly scopes).
+fn is_non_write_scope(scope: &str) -> bool {
+    scope.ends_with(".readonly")
+        || scope == "openid"
+        || scope.starts_with("https://www.googleapis.com/auth/userinfo.")
+        || scope == "email"
+}
+
+/// Returns true if the saved scopes are all read-only.
+pub fn is_readonly_session() -> bool {
+    load_saved_scopes()
+        .map(|scopes| scopes.iter().all(|s| is_non_write_scope(s)))
+        .unwrap_or(false)
+}
+
+/// Check if a requested scope is compatible with the current session.
+///
+/// In a readonly session, write-scope requests are rejected with a clear error.
+pub fn check_scope_allowed(scope: &str) -> Result<(), GwsError> {
+    if is_readonly_session() && !is_non_write_scope(scope) {
+        return Err(GwsError::Auth(format!(
+            "This operation requires scope '{}' (write access), but the current session \
+             uses read-only scopes. Run `gws auth login` (without --readonly) to upgrade.",
+            scope
+        )));
+    }
+    Ok(())
+}
+
 /// Handle `gws auth <subcommand>`.
 pub async fn handle_auth_command(args: &[String]) -> Result<(), GwsError> {
     const USAGE: &str = concat!(
@@ -275,6 +323,63 @@ async fn handle_login(args: &[String]) -> Result<(), GwsError> {
         ..Default::default()
     };
 
+    // If scopes changed from the previous login, revoke the old refresh token
+    // so Google removes the prior consent grant. Without revocation, Google's
+    // consent screen shows previously-granted scopes pre-checked and the user
+    // may unknowingly re-grant broad access.
+    if let Some(prev_scopes) = load_saved_scopes() {
+        let prev_set: HashSet<&str> = prev_scopes.iter().map(|s| s.as_str()).collect();
+        let new_set: HashSet<&str> = scopes.iter().map(|s| s.as_str()).collect();
+        if prev_set != new_set {
+            // Revoke the old refresh token so Google removes the prior consent grant.
+            if let Ok(creds_str) = credential_store::load_encrypted() {
+                if let Ok(creds) = serde_json::from_str::<serde_json::Value>(&creds_str) {
+                    if let Some(rt) = creds.get("refresh_token").and_then(|v| v.as_str()) {
+                        let client = reqwest::Client::new();
+                        match client
+                            .post("https://oauth2.googleapis.com/revoke")
+                            .form(&[("token", rt)])
+                            .send()
+                            .await
+                        {
+                            Ok(resp) if resp.status().is_success() => {}
+                            Ok(resp) => {
+                                eprintln!(
+                                    "Warning: token revocation returned HTTP {}. \
+                                     The old token may still be valid on Google's side.",
+                                    resp.status()
+                                );
+                            }
+                            Err(e) => {
+                                eprintln!(
+                                    "Warning: could not revoke old token ({e}). \
+                                     The old token may still be valid on Google's side."
+                                );
+                            }
+                        }
+                    }
+                }
+            }
+            // Clear local credential and cache files to force a fresh login.
+            let enc_path = credential_store::encrypted_credentials_path();
+            if let Err(e) = std::fs::remove_file(&enc_path) {
+                if e.kind() != std::io::ErrorKind::NotFound {
+                    return Err(GwsError::Auth(format!(
+                        "Failed to remove old credentials file: {e}. Please remove it manually."
+                    )));
+                }
+            }
+            if let Err(e) = std::fs::remove_file(token_cache_path()) {
+                if e.kind() != std::io::ErrorKind::NotFound {
+                    return Err(GwsError::Auth(format!(
+                        "Failed to remove old token cache: {e}. Please remove it manually."
+                    )));
+                }
+            }
+            eprintln!("Scopes changed — revoked previous credentials.");
+        }
+    }
+
     // Ensure openid + email scopes are always present so we can identify the user
     // via the userinfo endpoint after login.
     let identity_scopes = ["openid", "https://www.googleapis.com/auth/userinfo.email"];
@@ -351,6 +456,10 @@ async fn handle_login(args: &[String]) -> Result<(), GwsError> {
         let enc_path = credential_store::save_encrypted(&creds_str)
             .map_err(|e| GwsError::Auth(format!("Failed to encrypt credentials: {e}")))?;
 
+        // Persist the configured scope set for scope-change detection and
+        // client-side guard enforcement.
+        save_scopes(&scopes)?;
+
         // Clean up temp file
         let _ = std::fs::remove_file(&temp_path);
 
@@ -1167,6 +1276,16 @@ async fn handle_status() -> Result<(), GwsError> {
         }
     } // end !cfg!(test)
 
+    // Show configured scope mode from scopes.json (independent of network)
+    if let Some(saved) = load_saved_scopes() {
+        output["configured_scopes"] = json!(saved);
+        output["scope_mode"] = json!(if is_readonly_session() {
+            "readonly"
+        } else {
+            "default"
+        });
+    }
+
     println!(
         "{}",
         serde_json::to_string_pretty(&output).unwrap_or_default()
@@ -1179,10 +1298,11 @@ fn handle_logout() -> Result<(), GwsError> {
     let enc_path = credential_store::encrypted_credentials_path();
     let token_cache = token_cache_path();
     let sa_token_cache = config_dir().join("sa_token_cache.json");
+    let scopes_file = scopes_path();
 
     let mut removed = Vec::new();
 
-    for path in [&enc_path, &plain_path, &token_cache, &sa_token_cache] {
+    for path in [&enc_path, &plain_path, &token_cache, &sa_token_cache, &scopes_file] {
         if path.exists() {
             std::fs::remove_file(path).map_err(|e| {
                 GwsError::Validation(format!("Failed to remove {}: {e}", path.display()))
@@ -2208,4 +2328,37 @@ mod tests {
         let result = extract_scopes_from_doc(&doc, false);
         assert!(result.is_empty());
     }
+
+    // --- Scope persistence and guard tests ---
+
+    #[test]
+    fn test_save_and_load_scopes_roundtrip() {
+        let dir = tempfile::tempdir().unwrap();
+        let path = dir.path().join("scopes.json");
+        let scopes = vec![
+            "https://www.googleapis.com/auth/gmail.readonly".to_string(),
+            "openid".to_string(),
+        ];
+        let json = serde_json::to_string_pretty(&scopes).unwrap();
+        crate::fs_util::atomic_write(&path, json.as_bytes()).unwrap();
+        let loaded: Vec<String> =
+            serde_json::from_str(&std::fs::read_to_string(&path).unwrap()).unwrap();
+        assert_eq!(loaded, scopes);
+    }
+
+    #[test]
+    fn test_is_non_write_scope() {
+        // Readonly and identity scopes are non-write
+        assert!(is_non_write_scope("https://www.googleapis.com/auth/drive.readonly"));
+        assert!(is_non_write_scope("https://www.googleapis.com/auth/gmail.readonly"));
+        assert!(is_non_write_scope("openid"));
+        assert!(is_non_write_scope("email"));
+        assert!(is_non_write_scope("https://www.googleapis.com/auth/userinfo.email"));
+
+        // Write scopes are not non-write
+        assert!(!is_non_write_scope("https://www.googleapis.com/auth/drive"));
+        assert!(!is_non_write_scope("https://www.googleapis.com/auth/gmail.modify"));
+        assert!(!is_non_write_scope("https://www.googleapis.com/auth/calendar"));
+        assert!(!is_non_write_scope("https://www.googleapis.com/auth/pubsub"));
+    }
 }
diff --git a/src/main.rs b/src/main.rs
index bd72c642..976b51af 100644
--- a/src/main.rs
+++ b/src/main.rs
@@ -237,6 +237,11 @@ async fn run() -> Result<(), GwsError> {
     // to avoid restrictive scopes like gmail.metadata that block query parameters.
     let scopes: Vec<&str> = select_scope(&method.scopes).into_iter().collect();
 
+    // Enforce readonly session: reject write scopes if user logged in with --readonly
+    if let Some(scope) = scopes.first() {
+        auth_commands::check_scope_allowed(scope)?;
+    }
+
     // Authenticate: try OAuth, fail with error if credentials exist but are broken
     let (token, auth_method) = match auth::get_token(&scopes).await {
         Ok(t) => (Some(t), executor::AuthMethod::OAuth),